1. Field of the Invention
The present invention relates generally to a regulated power supply controller, and in particular to a feedback circuit for the controller of an isolated switching power supply.
2. Description of the Related Art
Power supplies are used in electronic devices to provide power to portions of the device""s circuit at specific voltage levels. Regulated power supplies are required where the voltage levels must be tightly controlled for proper operation of the circuit. A regulated power supply has a primary side to which electrical power, such as from an AC line or a higher DC voltage, is connected. The primary side is connected to the primary windings of a power transformer, and the secondary windings of the transformer are connected to the secondary side of the circuit to provide the regulated output voltage of the power supply. The output voltage may be, for example, a smaller DC voltage. The transformer provides an isolation barrier between the primary side and the secondary side.
Feedback is required to ensure the tight regulation necessary for controlling the output voltage level, and the feedback signal must be provided across the isolation barrier. An optical isolator is utilized to communicate the feedback signal across the isolation barrier.
A common type of regulated power supply is a switching power supply, in which regulation is performed by a square wave signal having a duty cycle that changes depending on the voltage to be output. The transmission of power from the primary side to the secondary side is switched on and off using the square wave, and the averaged value of the changing duty cycle rests in the regulated output voltage.
FIG. 1 shows a power supply circuit including a feedback circuit utilized to communicate the feedback signal across the isolation barrier between the primary side and the secondary side of the regulated power supply circuit. The feedback circuit includes a voltage divider made up of a resistor 10 and a resistor 12 which is provided across the output voltage Vout. The divided voltage is provided at a tap that is connected to a control lead of a reference element 14. In one embodiment, the reference element is a TL431 precision adjustable three terminal shunt voltage regulator, made by Texas Instruments. The reference element 14 is connected to a LED part 16 of an optical isolator 18, also referred to as an optical coupler, so that variations in the signal applied to the control lead result in changes in the light output by the LED part 16. The LED part 16 typically emits light in the infrared region, although visible light or other wavelengths are of course also possible,
Also linked to the control lead of the reference element 14 is an RC combination of a resistor 20 and a capacitor 22 linked to the output voltage Vout, and an RC combination of a resistor 24 and a capacitor 26 linked to the LED part 16. A resistor 28 is provided in series with the LED part 16 between the LED part 16 and the output voltage Vout.
The optical isolator 18 provides signal transmission across the isolation barrier 30, and the light, such as infrared light, from the LED part 16 is transmitted to a photodiode 40 of the optical coupler 18. The photodiode 40 is connected to a base of a transistor 32 within the optical isolator 18. The level of received light by the photodiode 40 is translated to current flow through the transistor 32 and fed to an output resistor 34. An output 36 to a control circuit 38 for the power supply is taken at the connection between the transistor 32 and the resistor 34.
The power supply that utilizes the feedback circuit is shown in simplified form, including the controller, or control circuit 38, which is a pulse width modulation controller. The output of the controller 38 is connected to the gate lead of a power FET 42. The FET 42 is connected to the primary of a transformer 43 which receives the input voltage for the power supply. The secondary side of the transformer 43 receives the transformed voltage switched by the pulse width modulator 38, forwards it through diodes 44 and 45, through an inductor 46 and through a capacitor 47 to produce the output voltage of the power supply at the outputs 48 and 49.
The feedback circuit, thus, operates through the control circuit 38 to cause regulation of the power supply to generate a desired output voltage. The output voltage is provided as the voltage Vout that is provided to the LED 16 of the optical isolator 18.
At startup of the circuit, the reference element 14 does not respond until the operating voltage has reached the desired operating point. The startup response of the reference element 14 causes an overshoot in the feedback voltage so that the output voltage overshoots the desired output voltage level. The circuit then overcorrects the output voltage, which results in ringing in the circuit during startup.
The present invention provides a power supply feedback circuit with a gradual startup to avoid ringing in the circuit. The present circuit includes a control connected to the reference element to reduce the output voltage to a lower level during startup. The control then gradually raises the output voltage to the desired operating level. In one example, the control circuit includes an active element and an RC combination connected to the control lead of the reference element.